Plastic packaging materials continue to gain acceptance by the packaging industry because they are tough, lightweight, and easily fabricated into containers. The deployment of plastic materials into films, trays, bottles, cups, bowls, coatings and liners is already commonplace. Although plastic materials such as polyethylene terephthalate (PET) and polyethylene naphthlate (PEN) offer the packaging industry many benefits with broad design flexibility, the utility of plastic materials is often limited in situations where preventing ingress of atmospheric gases (primarily oxygen) is necessary to assure adequate product shelf life. When compared to traditional packaging materials such as glass and steel, plastics often offer inferior barrier properties which limits their acceptability for use in packaging items that are sensitive to atmospheric gases, such as beer and fruit juices, particularly when the exposure to the atmospheric gases could result in spoilage. The packaging industry continues to seek packaging materials for use with oxygen sensitive products that offer the design flexibility of plastics with the inherent recycle advantage of plastics and at the same time have the barrier properties of glass and steel.
The use of multilayer bottles that contain core layers of an oxygen scavenging material is commonplace. Typically, the center layer is a blend of inorganic, or organic polymeric, oxygen scavenging material. Multilayer oxygen scavenging packages and walls for a package are disclosed in U.S. Pat. Nos. 5,021,515, 5,639,815, and 5,955,527 to Cochran. The multilayer packages of Cochran comprise inner and outer layers of a non-oxidizable polymer and a core layer that consists of an oxidizable polymer and a catalyst or polymer blends containing an oxidizable polymer and a catalyst. The oxidizable polymer is a polyamide such as MXD-6 nylon. Although oxidizable polymers such as MXD-6 could be used as a single layer package wall, Cochran teaches that the oxidizable polymer is preferably protected from undesired direct contact with the package contents and the environment by the inner and outer layers.
Blends of PET and MXD-6 in multilayer applications are also disclosed in U.S. Pat. No. 5,077,111 to Collette. Collette discloses a five layer preform wherein the inner, outer and core layer are formed of PET and the inner and outer intermediate layers are formed from a blend of PET and MXD-6. Similar to the bottles disclosed in Cochran, the oxidizable polymer, MXD-6, comprises the core layer and is encapsulated by PET in the multilayer container of Collette. This is because direct contact with the package contents may cause difficulties such as undesirable chemical reactions between the oxygen scavenger and the package contents or leaching of oxygen scavenger components or oxidation by-products into the package contents.
The multilayer containers of the prior art have several drawbacks. First, due to the dissimilar nature of the materials in the core and outer layers, they are not readily recyclable. Recycling for multilayer containers often requires separation of the barrier layer (nylon or EVOH) from the main portion of the recycling stream. Such separation is often incomplete and thus the barrier resin contaminates the recycle stream.
Second, it is both more difficult and more expensive to manufacture multilayer containers. Currently, there are few organizations that are capable of producing good quality multilayer barrier preforms. The difficulty is often achieving a uniform layer of barrier material since it may constitute as little as 4% of the total bottle weight. The barrier material must uniformly cover the total bottle surface in order to be effective. Any non-uniformity of the core layers leaves pathways through which oxygen may more readily permeate. It is recognized, however, that some multilayer fabricators may intentionally inject the scavenger at variable thicknesses throughout the part to increase the relative scavenger concentration in the thinnest areas of the final part. Third, to prevent contacting the food and beverage with the core layer material which may not be suitable for direct food contact, fabricators must precisely control placement of this material so that the food contact surface is pure PET. In order to do this molders must start and finish each molding cycle with pure PET. This leaves areas of the preform without coverage at the base and finish of the bottle through which oxygen can enter the container. Fourth, multilayer injection molding equipment is more difficult to operate, more expensive to purchase and maintain and is not as prevalent in the bottling industry as monolayer equipment which is currently used, for example, in soft drink applications.
Fifth, because the multilayer barrier bottles are made from dissimilar materials the interface between the materials is weak and the bottles often delaminate in use or when stressed during filling. This results in a wall that appears to be fractured. This is common with multilayer bottles that have an EVOH or an MXD6 nylon scavenger inner layer with outer layers of PET. Sixth, industrial scrap made from the multilayer bottle process cannot be directly ground up and re-used in the process for the same reasons that they are less desirable in the recycle stream. Finally, multilayer containers work against an oxygen scavenger's ability to eliminate headspace oxygen since the scavenging layer and the headspace are separated by the inert inner-most layer of the multilayer container which reduces the scavenger's effectiveness.
Monolayer packages of the prior art have drawbacks as well. The Collette '111 patent teaches monolayer blends of PET and MXD-6. But as discussed above, direct contact between the package wall and the package contents may cause undesirable migration of the oxygen scavenger or its by-products from the package wall into the package contents. This effects the taste of the contents and the appropriateness of the package for direct food contact applications. In addition, these blends require a minimum of 4% MXD-6 which leads to a loss of clarity (haze) in the final package because the two polymers are incompatible and phase separate. Because of the mixture of dissimilar materials, the PET/MXD-6 monolayer containers are not suitable for direct recycling and must be segregated and landfilled.
Thus, there is a need for an oxygen scavenging container that is readily recyclable, has substantially uniform distribution of the oxygen scavenger throughout the package wall, can be manufactured on conventional molding equipment, is not prone to delamination when stressed and has a low level of extractable components. A monolayer package comprised primarily of polyester yet possessing oxygen scavenging ability can satisfy this need.
One method proposed for providing oxygen scavenging ability to polyester is the incorporation of oxygen scavenging species into the polyester. Such incorporation would result in a modified polyester with enhanced oxygen barrier properties that could be formulated to meet the particular needs of the packaged product by varying the level of oxygen scavenging substance incorporated into the polyester. Naturally, increasing the oxygen barrier properties of packaging polyesters such as PET must be done without sacrificing the salient features and properties of PET. For the purposes of this invention, the salient features and benefits of PET include (1) transparency, (2) rigidity, (3) good passive barrier properties, (4) recycle capability, (5) processability, (6) reasonable cost, (7) a long history of use by the plastic packaging industry, and (8) does not adulterate the package contents. Thus, there are at least two separate considerations involved in development of materials and methods that could be used to improve the oxygen scavenging properties of PET. First, it is necessary to identify species which are readily oxidizable and possess high oxygen scavenging capacity so that their amount can be minimized. Logic dictates that use of the smallest amount of material should have the least impact on the other salient features of polyester packages as well as the lowest amount of extractable by-products. However, other considerations must be made in addition to oxygen scavenging capacity including such factors as cost, clarity, processability, recycling, etc. Second, it is necessary to devise a means for permanently incorporating the more promising scavenging species into the packaging and bottling polyesters to form desirable oxygen scavengers.
Applicants were able to satisfy both of these considerations by developing novel modified copolymer compositions comprising predominantly polycondensate segments and a lesser weight percentage of oxygen scavenging moiety (OSM) segments. These oxygen scavenging compositions can be incorporated directly into a monolayer container wall that has all the salient features of PET as well as the ability to scavenge both headspace oxygen and oxygen permeating through the container wall.